A rotational scanner is a mechanical-optical device which, when rotated about its axis, imparts an angular change in an incident optical beam. The scanner is comprised typically of a substrate having reflective optical facets. Of the two generic scanner types, pyramidal or prismatic, the pyramidal ones are the principal subject of this invention. This type is exemplified in U.S. Pat. Nos. 3,619,039 and 5,114,217. Wherein the stationary beam direction is essentially paraxial or radially symmetric with the spin axis, and the changing beam direction is typically near-perpendicular to the axis, and rotating angularly about it by the action of the scanner. In the case of laser beam scanning, the fixed input illuminating beam is usually paraxial (or converging to a point coincident) with the axis, and the redirected beam is rotated about the axis. The pyramidal scanner is identified by its facet surfaces being oriented at an angle to the axis (usually at angles greater than 10.degree.), forming a "pyramidal" set. A detailed description of these factors appears in the journal paper entitled, "Fundamental Architecture of Optical Scanning Systems" by Leo Beiser, in Applied Optics, Vol. 34, No 31, pp 7307-7317 (1 Nov. 1995), in which pyramidal forms are represented in FIGS. 2 and 4, and prismatic ones in FIGS. 3 and 10.
Often, the scanners are operated at such high speeds that the inertial forces developed within the rotating substrate exert sufficient stress to distort the optical surfaces which form the facets. When this effect is apparent, the geometric purity of the optical beam which is redirected by the distorted facet is perturbed, forming optical aberration. Consequently, the quality of a focal spot derived from such a distorted beam is also aberrated, degrading the important optical property known as resolution. It is an object of this invention to provide a means for reducing substantially --and even approaching the elimination of --the degrading effects of inertial deformation, without resorting to the costly use and fabrication of exotic substrate materials, such as beryllium. It is noteworthy that the use of such materials can only reduce the degrading inertial effects by a limited factor, while the method disclosed here provides means for actually balancing the error toward a null.
FIG. 1 shows the consequence of rotating an uncompensated scanner at high speed. Illustrated is a generic pyramidal scanner, section view through its axis 115. For development of the distortion process, rather than being formed of a single solid substrate, assume the scanner is composed of an array of adjacent thin discs 110 whose outer radii terminate on a locus which defines the facet surface. Upon rotation of the array about its axis, centrifugal "g-forces" will develop for each elemental mass m in each disc, in proportion to the relation .omega..sup.2 r, where .omega. is the rotational angular velocity and r is the local radius of each m. The original flat facet is shown at 102, and the concave distorted facet is shown at 104 (with distortion highly magnified). Assuming, first, that each disc is free to expand radially under this stress, those viewed from the apex (a) toward the base plane (b) will experience progressively greater expansion, in accordance with the increased radii of their mass elements m; each to its own destination, independently of its neighbors.
This simple model is complicated, however, by the real connection between each disc; effectively exerting shear between the discs as each tries to expand to its unique height. Thus, the gradual increase in height is accompanied by a progressive bending of the array "to the left", as the shorter neighbors constrain the greater expansion of the longer discs. The resulting concave deformation of the facet is confirmed in subsequent expression of finite element analysis (FEA) conducted on an actual scanner.